In a gas turbine engine, fuel is burned in a combustor to provide heat which expands propulsion gases to provide thrust. The engine can be accelerated by adding more fuel. However, the amount of fuel added must be precisely controlled in order to prevent a condition known as engine stall. Engine stall can occur, for example, at a time when the engine is at a relatively low speed. At this time the metallic elements which surround the combustor are relatively cool. Addition of more fuel can result in greater combustion, yet the heat generated by the combustion can be absorbed by these metallic elements instead of by the propulsion gases and, consequently, the increase in gas expansion will be relatively low and the engine may stall.
On the other hand, if the engine has been running at a relatively high speed for a time, and then temporarily dropped to a relatively low speed, if acceleration is attempted at this time by injection of added fuel, stall can occur but for different reasons: the previous high speed condition raised the combustor to a high temperature. The low speed condition ordinarily creates a low combustor temperature, but in this particular instance, the combustor metal has not yet cooled from the high temperature and it thus functions as a heat reservoir. When fuel is injected in the attempt to accelerate the engine, the burning fuel as well as the metallic combustor elements can inject heat into the propulsion gases. This excess heat can cause the engine to stall.
Further, under all conditions, there is a limit to the amount of fuel which can be injected into the combustor. If there were no limit imposed, when the pilot of an airplane requested instantaneous acceleration, the fuel control would otherwise inject an excessive amount of fuel into the combustor, probably stalling the engine.
A classical method of controlling acceleration to prevent stalls entails controlling the rate of fuel delivery as a function of engine speed. Stalls are reduced by programming error margins into the rate of delivery. That is, for example, the rate of fuel delivery to a cold combustor is limited by that which a hot combustor can tolerate under the same conditions. Thus, these margins prevent situations from occurring in which too little fuel is provided to a cold combustor and too much fuel is provided to a hot combustor. However, it is clear that the use of such margins prevents the attainment of the acceleration which is theoretically possible: the fuel supplied under a given set of operating conditions is limited by the error margins which are, in general, not responsive to the conditions prevalent at a given time.
Further, no two engines are identical and they will thus accelerate differently in response to the same amount of fuel supplied to each. It is desirable to sense and control the acceleration of the engines because excessive acceleration can cause thermal cycling which reduces the lifetimes of the engines. Sensing acceleration poses a problem because the speed signal generally contains a high frequency noise component which is amplified when the time derivative is taken to calculate acceleration. Thus, direct computation of the acceleration sought to be controlled presents difficulties.
Still further, many engine fuel controls commonly utilize a static compressor discharge pressure (P.sub.3) as an input parameter. This poses at least two problems: one, the signal produced by the pressure transducer used is generally an analogue signal and must be digitized; and two, pressure transducers which are accurate over the necessary range of pressures (about 10 to 350 psi) are expensive.